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EURAMET project 1242 underlines the need for research in the field of optical surface measurements

Categories:
  • Division 5
  • Fundamentals of Metrology
22.12.2020

Within the framework of the EURAMET project 1242, coordinated by PTB, comparative measurements of areal roughness with different optical microscopes were carried out for the first time at six national metrology institutes.

Roughness measurements were performed on four samples. Three samples, which demand a rather high spatial resolution but a rather small FOV, are two lappened Si chips (ARS-F1 and ARS-F2) from Simetrics and one ultrafine roughness standard (UFRS) produced by focused ion beam (FIB) from point electronic. The fourth sample (B40-VP04) from Rubert & Co. in England with very longwave-features, thus a much larger measurement field is required to reliably determine the roughness.

Furthermore, measurements were performed on a resolution standard of type RS-N from Simetrics, which has rectangular profiles with the height (240 nm) but different grating periods in the range of 300 nm to 6 µm, to get some rough information about the instrument transfer function.

The comparison revealed that especially the finer roughness standards with comparatively short wavelength roughness cause unexpectedly great difficulties in optical roughness measurement. The values reported by the individual institutes, e.g. for the roughness Sa, deviated from the reference values by up to 60 %, and the uncertainties reported by experienced metrologists proved to be too small. In addition, the measurement results show a large dependence on the selected measurement principle and the objectives used. The preliminary analysis shows that the spatial frequency spectrum, the flank angles of the surface texture have a significantly greater influence on the roughness results than previously assumed.

The comparison also underlines that if only rectangular gratings with different periods are used, the transfer behavior of an instrument for different slopes cannot be characterized. Therefore, sinusoidal structures (see short the short report on determining topographic spatial resolution) must be used in the future. It is suspected that the artifacts obtained by optical measurements lead to strong deviations of the measured roughness characteristics. However, more experimentally investigations are needed.

The challenge for the very long wavelength standard (B40-VP04) was to validate stitching methods implemented in the instruments. Here, the selected stitching algorithm, leveling strategy, and preprocessing methods chosen have influence on the poor consistency of the roughness parameters determined by the participants. However, a re-evaluation of the raw data provided by the participants with an evaluation software developed by PTB showed that the measurement results of the participants agreed very well.

A follow-up project is therefore applied for within the framework of EMPIR call 2020 “Metrology for Industry” to investigate the influence factors arising from the measuring principles, hardware setup and feature properties by closely linking experimental investigations and numerical modelling together with other research partners. The overall objective of the project is to enable traceable 3D roughness and dimensional measurements using optical 3D microscopy and optical distance sensors, with a special focus on the development of practical instructions (Good Practice Guides) for the selection of most suitable measuring devices and their settings for the respective measurement purpose.


Fig. 1: Measured roughness values of the participants for the silicon fine roughness sample ARS F1 (red line: weighted mean value, green line: PTB-AFM value, CF: confocal microscope, VSI: white light interferometry).

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